1. Field of the Invention
The present invention relates generally to programmable electronic digital thermostats for use with heating and cooling systems, and more particularly to an improved programmable electronic digital thermostat that provides improved control of the heating and cooling system for varying load conditions in order to maintain the ambient temperature within a predetermined range of the scheduled set temperature while minimizing ambient temperature overshoots and undershoots and that includes an improved power supply circuit for simply and effectively providing its operating power utilizing conventional existing wiring.
2. Description of the Prior Art
Known electronic digital thermostats provide substantial energy savings by controlling conventional temperature conditioning systems in accordance with selected temperatures for a sequence of time intervals.
For example, U.S. Pat. No. 4,298,946, issued Nov. 3, 1981 entitled "Electronically Controlled Programmable Digital Thermostat" discloses a programmable thermostat for maintaining desired temperatures during selected time periods and includes programmed instructions for measuring actual time rate of change of the ambient temperature during heating and cooling cycles to anticipate reaching the desired temperature, for delaying a state change (activate or deactivate) of the heating and/or cooling for a delay time period; and for defining a hysteresis to prevent unnecessary reaction to small changes in the ambient temperature.
U.S. Pat. No. 4,388,592, issued June 14, 1983 entitled "Electronically Controlled Programmable Digital Thermostat Having Variable Threshold Hysteresis With Time" discloses a similar thermostat further including programmed instructions for varying a threshold hysteresis temperature with time in discrete steps around the desired ambient temperature or set temperature. In this disclosed thermostat, each time a switch, such as a triac, for controlling the operation of a temperature condition system changes state, the threshold hysteresis temperature is alternately changed between a positive and negative initial value around the set temperature. This initial value of the threshold hysteresis temperature is maintained for a predetermined fixed time period. The threshold hysteresis temperature is then reduced in a discrete step toward the set temperature unless the measured ambient temperature reaches the initial hysteresis temperature during the fixed time period so that the effective duty cycle rate of the temperature conditioning system is adjusted.
U.S. Pat. No. 4,442,972, issued Apr. 17, 1984 entitled "Electrically Controlled Programmable Digital Thermostat And Method For Regulating The Operation Of Multistage Heating And Cooling Systems" discloses a thermostat similar to that described in U.S. Pat. No. 4,388,592, further including programmed instructions for determining a restart time prior to the next scheduled time interval for initiating operation of the temperature conditioning system to achieve the next temperature at the scheduled time. This patent discloses routines for energy efficient control of a multistage temperature conditioning system that determine whether or not an actual rate of change of zone temperature effected by operation of the main system stage is below a predetermined selected rate of temperature change. An auxiliary system stage is operated only if necessary to achieve the predetermined rate of temperature change.
While such thermostats have provided significantly improved performance as compared to conventional electromechanical thermostats, it is desirable to provide a programmable electronic digital thermostat that provides improved control of the temperature conditioning system for varying load conditions for maintaining ambient temperature within a predetermined desired range of the scheduled set temperature and particularly that avoids thermal overshoots and undershoots, that includes a power supply circuit utilizing an external power source of the temperature conditioning system for simply and effectively providing its operating power, and that includes a temperature detecting circuit for providing a pulse width modulated temperature signal.